The O
+ density data resulting from the ISS-b ion mass spectrometer measurements have been analyzed to obtain the global distribution of the nighttime O
+ densities at a height of 1, 100km and its seasonal variations. The results of the analysis show the existence of distinct O
+ density depression regions in mid-latitudes in the winter hemisphere, two depressions in the northern hemisphere and one depression in the southern hemisphere.
The O
+ density depressions are coincident with the regions of large downward plasma flow identified by the calculation of the worldwide pattern of vertical plasma motion induced by the neutral winds. And the regions of high or low O
+ density are closely connected with the regions with westward or eastward declination of the geomagnetic field in the northern hemisphere, respectively, and vice versa in the southern hemisphere. In addition, the O
+ densities in the depression regions in winter season are maintained at rather low levels till just before the equinox, and increase rapidly to the level near the summer values just after the equinox.
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